A number of in vitro model systems have been used to study the effects of exogenous and endogenous agents on vaginal tissue. Although useful, each model system developed to date has suffered from significant drawbacks.
Vaginal epithelial cells in monolayer culture have been used for basic biochemistry studies and to determine the effects of hormones, growth factors and exogenous agents on cervical and vaginal tissue. Others have used monolayer cultures of cervical or vaginal cells, in conjunction with blood derived dendritic cells and T-cells in suspension culture, to investigate the mechanism of infection involved in sexually transmitted diseases. However, cells in monolayer culture lack the differentiated function and barrier properties of cells found in normal tissue. Hence results with monolayer or suspension cells may not apply to normal human cervical-vaginal tissue.
A three dimensional tissue produced from cultured skin keratinocytes has also been used to study the effects of vaginal microbicidal agents. Although useful, due to the skin origin of the cells, the tissue is not fully representative of in vivo cervical and vaginal tissue, and thus its use as a vaginal tissue model is severely limited.
Multi-layered organ cultures of vaginal tissue explants were reported by Sobel et al. (Sobel, J. D., et al., In vitro, 15, 993-1000 (1979)) as early as 1979 when non-malignant vaginal tissues were cultured on glass slides. Other studies have used epithelial cell outgrowths from cervical and vaginal tissue explants to study pathogenesis of the in vivo tissue and the efficacy of microbicidal agents at preventing infection. However, the availability of normal (non-cancerous) human vaginal and cervical tissue is limited, and the inability to store such tissue for long periods, make organ cultures usable only in small academic research environments. Also, tissue explants are difficult to handle and manipulate and the problem of tissue by-pass is hard to avoid. For example, it is difficult to insure that applied agents do not circumnavigate the tissue by diffusing around the tissue edges.
In recent years, there has been a growing awareness that the experimental use of vaginal and cervical tissue may further be critical to the study of HIV infection. A number of studies have suggested that Langerhans cells found in the vaginal epithelium, which express CD4 receptors and other co-receptors, are the initial target for HIV-1 infection, and that infected Langerhans cells transmit HIV to CD4+ T-cells (Cohen, M. S., Lancet, 351 (supp III), 5-7 (1998)). Langerhans cells have also been shown to be important reservoirs for HIV/SIV replication in vivo (Hu, J., et al., Lab. Invest., 78, 435-451 (1998)). However, studies, such as those which indicate that HIV replication only occurs in the presence of T-cells (Granelli-Piperno, A., Steinman, R. M., et al., Curr. Biol., 14, 21-29 (1999).), and reports of direct HIV-1 infection of ectocervical-vaginal cells which are CD4− (Tan, X., Phillips, D. M., Arch. Virol., 141, 1177-89 (1996)), preserve controversy as to the role of Langerhans cells in HIV infection.
Development of an in vitro model system that is highly reflective of in vivo cervical and vaginal tissue would greatly facilitate the accurate determination of the effects of exogenous and endogenous agents on vaginal tissue and also the pathogenesis and prevention of sexually transmitted diseases.